The methods for the study are alarmingly straight forward. To calculate the average sales-weighted fuel economy, which is what that 24.9-mpg figure represents for August 2013, researchers merely calculated the monthly sales for each individual model line and the combined EPA fuel economy listed in the EPA Fuel Economy Guide. If there were multiple fuel economy figures for a model, like, say a Ford F-150, researchers used the average of the combined EPA ratings. Low-volume exotics and luxury car manufacturers, like Rolls-Royce or Ferrari, had sales and fuel economy for their entire model range calculated to come up with one number.
Both UofM researchers that conducted the study, Michael Sivak and Brandon Schoettle, claim that data indicates that the average should only climb, and that it's very likely that the next model year will be even better than the previous. Interestingly, one of the biggest jumps in the study has been the 1.2-mpg jump, from model year 2012 to MY2013.